Gate valves have long been provided with an internal gate chamber and passages through the body communicating with the gate chamber which passages are normally aligned with each other. In order to support the gate and to provide a seal between the body and the sides of the gate surrounding the passages, annular bushings have been provided and positioned within shallow recesses surrounding the openings of the passages into the gate chamber.
When the gate is closed, i.e., blocking flow between the passages, the pressure on the gate causes it to be urged in the downstream direction or toward the outlet passage. This brings the downstream bushing into direct metal-to-metal sealing contact with both its recess surface and the surface of the gate. It also moves the gate in a direction away from the upstream bushing which relaxes the loading on the upstream bushing. It is desired that the upstream bushing be provided with seals which ensure sealing between the bushing and the body so that such sealing surfaces are maintained sufficiently clean for subsequent metal-to-metal sealing and also to load the bushing against the gate for sealing.
In prior gate valve structures, attempts have been made to utilize a solid PTFE (polytetrafluoroethylene) square seal design for the body-bushing seal. These seals initially perform well until exposed to field conditions of pressure loading and temperature variations. Under such conditions the solid square PTFE seals have taken permanent deformation from plastic flow which results in loss of seal capability and allows contaminant buildup behind the bushings. Also, if the PTFE seals are restricted from deforming by tight groove dimensions, the seal on the upstream side will extrude when pressure is reversed, which results in the prevention of a metal-to-metal seal between the bushing and the body and thus creating a leak path.
PTFE O rings have problems similar to the solid PTFE seals. The O rings require some means for preloading and for maintaining contact of the bushing against the body. This design is limited to gate valves which have cavity clearances allowing for a wedging mechanism to preload and maintain such contact.
Spring loaded lip seals of PTFE and similar materials have been tested and problems with functioning as a unidirectional seal were encountered. These designs tested have resulted in the seal extruding and collapsing the internal springs when pressure is applied to the heel of the seal, i.e., reversed pressure from its normal sealing direction.
Metal O ring designs have been tested but they have problems in maintaining a pressure tight seal through pressure and temperature cycling. They also require extremely high preload forces which must be maintained during the function of the valve.
U.S. Pat. No. 3,114,561, discloses a composite seal ring composed of a U-shaped body of an inert material, such as polytetrafluoroethylene and an internal spring of stainless steel which is also U-shaped and provided with slots which extend across the top of the U to make it more flexible. In one form of the invention, the spring has two continuous edges and in another form it has only one continuous edge. The purpose of the seal is to provide sealing in an environment in which the fluids attack seals which are not primarily inert. The spring is used because the seal material is not resilient.
U.S. Pat. No. 4,487,393 discloses a wedge type of gate valve in which the valve member or gate is made up of two pieces and on closing one member engages a stop and the other member continues moving with the tapered surfaces between the two members causing the members to be wedged outward from each other into tight sealing engagement with their respective bushings or seat rings. The upstream bushing includes a resilient solid ring engaged between the ring and the gate and both bushings include resilient O rings sealing between the bushings and the body.
U.S. Pat. No. 4,483,511 discloses a ball valve having separate upstream and downstream seat rings with annular springs urging the rings into seating engagement with the ball and an annular seat (or seal ring) positioned in the seat ring to engage the ball. The seals between the seat rings and the body are provided by O rings.
U.S. Pat. No. 4,629,161 discloses a gate valve which uses O rings to seal bushings or seat rings against the radial surfaces of the valve body seat recesses.
U.S. Pat. No. 4,364,544 discloses a gate valve which uses a spring mounted sediment guard to prevent foreign material from depositing between the radial surface of the seat recess and the seat.
Additional sealing structures used with gate valve bushings are illustrated in U.S. Pat. Nos. 2,606,740 (special resilient seal on the exterior of the bushing and extending between the gate and the body), 3,033,224 (sealant supplied to grooves in gate and bushings) and 4,531,711 (O ring seals).
A valve similar to the valve of the present invention is shown in the Composite Catalog of Oilfield Equipment and Services 1986-87, pages 1015.